This invention relates to an optical disc drive and a method of controlling the power of laser light in a recording to an optical disc. In particular, this invention relates to gain control of a detector that detects the laser light to control the power of a laser light source.
Optical disc drives have a variety of applications, for example, in motion picture players, motion picture recording players, and data storage devices for computers. CD (Compact Disc), DVD (Digital Versatile Disc), and BD (Blu-ray Disc) are known as the kinds of optical disc for storing data. The CD, DVD, and BD each have a disc type for read only and a disc type for read and write.
An optical disc drive irradiates an optical disc with laser light emitted from a laser light source and reads data recorded on the optical disc with the laser light reflected off the recording surface of the optical disc. An optical disc drive having a recording function irradiates the recording surface of an optical disc with laser light to change the state of the recording surface, which results in recording data onto the optical disc.
For accurate and stable recording and reading, the power of laser light (laser output power) should be controlled with accuracy. For this reason, an optical disc drive includes a system that automatically controls the laser light power. This is called APC (Automatic Power Control). The APC monitors the power (intensity) of the laser light from the laser light source and adjusts drive current supplied to the laser light source so that the laser light power falls within a desired range.
The power of laser light is required to be controlled accurately and stably for proper servo control as well. An optical disc drive positions a laser spot precisely at a target position (positioning control of laser spot) through servo control of a pickup (optical head). The servo control system in the optical disc drive performs tracking servo control, which is positioning control in the disc radial direction, and focusing servo control. Since the servo control system uses (the RF signal of) the reflection of laser light off the recording surface like in reading user data, accurate and stable control of the laser luminance power is required for accurate and stable servo control.
The APC receives a part of the light thrown by the laser light source to an optical disc at a photodiode element and converts the light signal into an electric signal. This electric signal is amplified by an amplifier and transferred to a controller. The circuit module (laser-light detector) including the photodiode element and the amplifier is called monitor diode in this description.
The power of laser light applied to an optical disc is different depending on the kind of optical disc and whether the laser light is for a recording operation or a reading operation. Accordingly, the power of the laser light received by the photodiode element also varies depending on them. In the case where the gain of (the amplifier of) the monitor diode is fixed, the output from the monitor diode varies significantly with the intensity of the laser light. For this reason, APC has been proposed that alters the gain of the monitor diode depending on the laser light power (for example, refer to JP 2004-146050 A).